[LeetCode 156] Binary Tree Upside Down

最新推荐文章于 2021-08-08 02:32:16 发布

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本文介绍了一种方法，通过使用栈来翻转特定类型的二叉树，并将其转换为一种新的结构，其中原来的右节点变为左叶子节点。以给定的二叉树为例，展示了从原始树到翻转后的树的详细步骤。

Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.

For example:
Given a binary tree {1,2,3,4,5},

return the root of the binary tree [4,5,2,#,#,3,1].

solution:

use stack to store left node from top to bottom, then reverse to construct the tree.

if(root ==null || (root.left == null && root.right == null)) return root;
        Stack<TreeNode> path = new Stack<>();
        while(root.left!=null) {
            path.add(root);
            root = root.left;
        }
        path.add(root);
        while(!path.isEmpty()) {
            TreeNode temp = path.pop();
            if(!path.isEmpty()) {
                temp.right = path.peek();
                temp.left = path.peek().right;
            }else{
                temp.right = null;
                temp.left = null;
            }
        }
        return root;